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Superconductivity and Phase Stability of Potassium-Doped Biphenyl† Cite This: Phys PCCP View Article Online PAPER View Journal | View Issue Superconductivity and phase stability of potassium-doped biphenyl† Cite this: Phys. Chem. Chem. Phys., 2018, 20,25217 Guo-Hua Zhong, a Dong-Yu Yang,b Kai Zhang,b Ren-Shu Wang,b Chao Zhang,c Hai-Qing Lin*d and Xiao-Jia Chen*b Phenyl molecules are proposed as potential high-temperature superconductors due to exhibiting interesting properties. Here, we report the discovery of superconductivity with the critical temperature (Tc)ofB7.2 Kelvin in potassium (K)-doped biphenyl (C12H10). The dc magnetic susceptibility measurements provide solid evidence for the presence of the Meissner effect in KxC12H10. The Raman spectra detected Received 14th August 2018, bipolaronic characteristics in this superconducting state, which are proposed to account for the electron Accepted 6th September 2018 pairing. Theoretical simulations provided the information of the crystal structure of KxC12H10. Combining DOI: 10.1039/c8cp05184d XRD data with formation energy, we suggest that the superconducting phase corresponds to K2C12H10 or with a small charge fluctuation in a layered structure. The discovery of superconductivity in K-doped rsc.li/pccp biphenyl vastly expands the potential applications in the superconducting field. 8 1 Introduction under 1.2 GPa. Then, in 2003, the Tc was raised to 13.4 K in the BEDT-TTF salt family under 8.2 GPa.9 Recently, an inter- Organic materials have attracted increasing interest because of esting superconductivity has been discovered in polycyclic their many fascinating phenomena such as low dimensionality, aromatic hydrocarbons (PAHs) doped by alkali metals.10–15 strong electron–electron and electron–phonon interactions and The PAH materials exhibit the characteristics of multi-Tc the proximity of antiferromagnetism, insulator states and phases and the highest value of Tc increases with the number superconductivity. Especially, organic based compounds were of benzene rings, reaching 33 K in K-doped 1,2:8,9-dibenzo- suggested as candidates for high temperature or room tempera- pentacene.13 In addition, researchers observed that the poly- ture superconductors since the electrons can interact with much (para-phenylene) (PPP) materials, such as K-doped p-terphenyl,16–20 higher excitation energy than the phonon energy in these p-quaterphenyl,21,22 and p-quinquephenyl,23 which are non-edge- Published on 07 September 2018. Downloaded by Trinity College Dublin 10/12/2018 6:12:16 AM. materials.1,2 Numerous studies have been done over a half shared aromatic hydrocarbons, also possess rich superconduc- century. In 1965, the first carbon-based superconductor C8A tivities. It is worth mentioning that p-terphenyl was found to be 19 (A represents the alkali metal) was studied, exhibiting a super- superconducting with Tc as high as 123 K, which is comparable 3 conducting transition temperature (Tc) of 0.02–0.55 K. In 2005, to the record high Tc of about 130 K in some cuprate super- 24 the Tc values of graphite-like superconductors were raised to conductors at ambient pressure. This discovery of high Tc 4,5 6.5 and 11.5 K for YbC6 and CaC6, respectively. In 1991, superconductivity in p-terphenyl seemingly supports the above superconductivity was also observed in the complete carbon idea suggested by Little and Ginzburg. B 6 material of fullerene by doping potassium (K), with Tc 18 K. Interestingly, the low-Tc phase with Tc in the range of 5–7 K 7 10–15 Furthermore, the Tc in cesium-doped fullerene reached to 38 K. exists in observed PAH superconductors, though some In 1980, a real organic superconductor consisting of C and H PAHs exhibit the characteristics of multi-Tc phases. By explor- atoms, di-(tetramethyltetraselenafulvalene)-hexauorophosphate, ing the superconductivity of K-doped solid benzene, which is a named (TMTSF)2PF6, was reported with Tc equaling 0.9 K basic unit of aromatic compounds (the electron–phonon cou- pling constant l = 0.67 and Tc = 6.2 K), theoretical studies a Shenzhen Institutes of Advanced Technology, Chinese Academy of Sciences, pointed out that the low Tc phase of 5–7 K is a common feature Shenzhen 518055, China for all aromatic hydrocarbon superconductors and is easy to 25 b Center for High Pressure Science and Technology Advanced Research, obtain due to better stability. The subsequent experimental Shanghai 201203, China. E-mail: [email protected] investigations confirmed the observation, for example, 7.2 K in c Department of Physics, Yantai University, Yantai, 264005, China 17 22 23 d p-terphenyl and p-quaterphenyl, 7.3 K in p-quinquephenyl, Beijing Computational Science Research Center, Beijing, 100193, China. 0 26 27 E-mail: [email protected] 7.2 K in 2,2 -bipyridine, and 3.5/7.2 K in triphenylbismuth. † Electronic supplementary information (ESI) available: Atomic coordinates and The p electron network plays an important role in the super- bond lengths for pristine and doped biphenyl. See DOI: 10.1039/c8cp05184d conductivity of PAH and PPP compounds and drives a strong This journal is © the Owner Societies 2018 Phys. Chem. Chem. Phys., 2018, 20, 25217--25223 | 25217 View Article Online Paper PCCP couplingofthechargecarrierstothelattices.Theelectron– 2 Experimental and phonon interactions can account for the superconductivity of computational methods this unified superconducting phase.25,28 Within the frame- work of BCS theory, an increase of doping concentration may 2.1 Experimental details result in a stronger electron–phonon interaction, which is High quality potassium and crystalline biphenyl were pur- 29 helpful for obtaining a higher Tc, and the pressure can also chased from Sinopharm Chemical Reagent and Alfa Aesar, 30 drive a higher Tc. respectively. In our experiments, the doped biphenyl was However, the electron–phonon interaction is not enough synthetized by heating a mixture of small pieces of fresh to produce the high Tc superconductivity such as 18 K potassium and biphenyl with a mole ratio of 3 : 1 at a tempera- in picene,10 15 K in coronene,11 33 K in 1,2:8,9-dibenzo- ture of 1201–130 1C for 48 hours. All the synthesis procedures pentacene,13 and 43/123 K in p-terphenyl.18 Other factors were performed in a glove box with moisture and oxygen levels mayplayimportantrolesinthehighTc superconductivity. less than 0.1 ppm, and the sample was sealed in a quartz tube Keyfeaturesoforganicsuperconductorsarethelowdimen- under a high vacuum of better than 1 Â 10À4 Pa in the heating sionality of the materials and strong electron–electron corre- process. The dc magnetization measurements were performed lation. The fulleride Cs3C60 was found to be a true Mott in the temperature range from 1.8 K to 300 K by a commercial insulator because of strong electronic correlation effects,31 Magnetic Property Measurement System (Quantum Design). In though it could be turned into a superconductor by pressure.7 the Raman scattering experiments, a 660 nm excitation was Superconductivity induced by pressure in Cs3C60 was found applied to obtain the Raman spectra of the pristine and doped 32 to be related to the antiferromagnetic Mott insulator. K3 biphenyl. The laser power was selected as less than 2 mW before picene was also revealed as a strongly correlated electron a Â20 objective to avoid possible damage of samples. The X-ray system,33,34 which results in the absence of metallicity.35 diffraction experiments were performed in the 2-theta range of Theoretical calculations predicted that K-doped PAHs in their 51–601 by an X-ray diffractometer (Empyrean, PANalytical B.V.). ground state exhibit near antiferromagnetic behaviour.34,36,37 The samples were put on specific glasses and then covered with Especially for K-doped p-terphenyl, the bipolaronic thin films to isolate the air. characteristic has been identified from Raman scattering 17–19 measurements. Photoemission spectroscopy conducted 2.2 Computational details on surface K-doped p-terphenyl crystals showed a gap persist- ing up to at least 120 K that shares a similar temperature To study the structural and electronic properties of pristine and K-doped biphenyl, we employed the Vienna ab initio simulation dependence to the obtained spectra of Bi–Sr–Ca–Cu–O 41,42 superconductors.38 Furthermore, an analogous gap below package (VASP) based on the projector augmented wave 50 K was also found in K-doped p-terphenyl films fabricated method. For the planewave basis-set expansion, an energy cut- 39 off of 600 eV was adopted. The Monkhorst–Pack k-point grids on Au(111). All these features indicate that the high Tc superconductivity of organic materials with p-electron net- were generated according to the specified k-point separation of 0.02 ÅÀ1 and the convergence thresholds were set as 10À6 eV in works cannot simply be explained by the electron–phonon À3 À1 mechanism. This indicates that both the electron–phonon energy and 10 eV Å in force. The generalized gradient form Published on 07 September 2018. Downloaded by Trinity College Dublin 10/12/2018 6:12:16 AM. interaction and electron–electron correlations work together (GGA) of the exchange–correlation functional (Perdew–Burke– Ernzerh of 96, PBE) was adopted.43 Considering the non-local to enhance the Tc of aromatic hydrocarbons with increasings 25 interactions, we added the correction of van der Waals (vdW) numbers of benzene rings. 44 Due to the appearance of superconductivity and possible in the version of vdW-DF2 in the calculations. The necessity of the vdW-DF2 functional has been confirmed by our previous high Tc, PPP superconductors provide a good system to study 29,45 the interplay of strong electron–electron and electron–phonon studies. interactions in a low-dimensional system. Comparing with p-terphenyl, biphenyl (C12H10) is the shortest polymer with only two phenyl rings connected by a single C–C bond.
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